In electrophotography, an organophotoreceptor in the form of a plate, disk, sheet, belt, drum or the like having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of a photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas where light strikes the surface, thereby forming a pattern of charged and uncharged areas, referred to as a latent image. A liquid toner or solid toner can then be provided in the vicinity of the latent image, and toner droplets or particles can be deposited in either the charged or uncharged areas, depending on the properties of the toner, to create a toned image on the surface of the photoconductive element. The resulting toned image can be transferred to a suitable ultimate or intermediate receiving surface, such as paper, or the photoconductive element can operate as the ultimate receptor for the image. The imaging process can be repeated many times to complete a single image, which can involve, for example, overlying images of distinct color components or effecting shadow images to complete a full color complete image, and/or to reproduce additional images.
Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge generating compound and a charge transport material selected from the group consisting of a charge transport compound, an electron transport compound, and a combination of both are combined with a polymeric binder and then deposited on the electrically conductive substrate. In the multilayer embodiments based on a charge transport compound, a charge transport compound and a charge generating compound are in the form of separate layers, each of which can optionally be combined with a polymeric binder, deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the “dual layer” arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the “inverted dual layer” arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to generate charge carriers (i.e., holes and/or electrons) upon exposure to light. The purpose of the charge transport material is to accept these charge carriers and transport them through the charge transport layer in order to discharge a surface charge on the photoconductive element. The charge transport material can be a charge transport compound, an electron transport compound, or a combination of both. When a charge transport compound is used, the charge transport compound accepts the hole carriers and transports them through the layer in which the charge transport compound is located. When an electron transport compound is used, the electron transport compound accepts the electron carriers and transports them through the layer in which the electron transport compound is located.